The invention relates to a flow machine for a fluid, comprising a radial sealing gap between stator parts and a rotor, with a self-supporting wear ring, of which the outer jacket surface is provided with a wear surface and which is axially fixed in the direction of the axis of rotation of the rotor, being provided at the sealing gap.
In flow machines, radial sealing gaps which are axially flowed through frequently arise between the rotating parts and the stator parts and must be kept small in order to keep leakage losses low. Many fluids are provided with solid parts which can lead to abrasive wear in such narrow sealing gaps. This is caused on the one hand by the rotation of the rotor and on the other hand by a pressure difference which is effective over the sealing gap in the axial direction. A widening of the sealing gap increases the loss flow and reduces the volumetric efficiency.
Up to a certain quota of solid parts, one resorts to coating the surfaces in the sealing gap with a wear-resistant protective coating, the lifetime of which is given through the nature of the operation. When the repair intervals become too short, therefore, there remains only the use of self-supporting wear rings of ceramic materials. These admittedly have a very high wear resistance, but are however constructionally difficult to handle, since they are rather brittle and can be manufactured only in simple shapes due to their wear resistance. A further disadvantage in comparison with other materials consists in their insufficient coefficients of thermal expansion in connection with their low elasticity, which represents a risk for the operator of a flow machine, such as for example a multistage radial pump, when a wear ring is stretched at its inner side. Temperature fluctuations in the flow medium, but also certain operating situations such as the forwarding against a closed slider, which can likewise lead to a temperature increase, can lead to impermissible tension stresses in a wear ring of this kind.
In a wear ring which is stretched at its outer jacket surface and the wear surface of which lies on the inner side, a lowering of the temperature results in a uniformly distributed increase of the compression stress, which is as a rule permissible. If rather higher temperatures are operated at, the outwardly held wear rings can be held in a shrink connection, which in spite of the greater thermal expansion of a holding metal part is still sufficient for the centering and the force transfer. The situation is different in wear rings which are provided with a wear surface on their outer jacket surface.
It is an object of the invention to make wear rings with low coefficients of thermal expansion which are provided on their jacket surface with a wear surface in order to make them usable for flow machines in a large range. This object is satisfied in that the wear ring consists of a material with a lower thermal expansion than parts on which it is supported; in that it lies hollowly on its inner jacket surface up to a specific temperature intended for the sealing gap; and in that a first conical holder surface encounters with the apex of its cone a second holder surface, which is designed as a straight shoulder, in the plane of the latter on the axis of rotation or encounters a second holder surface, which is designed as a conical holder surface, at its cone apex on the axis of rotation.
This arrangement has the advantage that the conical holder surfaces, which expand to a greater extent than the wear ring when the temperature increases, expand along their cone jacket lines relative to the wear ring. If the axial bias force on the cone surfaces is not chosen too largexe2x80x94the wear ring also expands somewhatxe2x80x94then a minimum sliding movement between the conical surfaces can take place, which prevents excessive ring tensions in the form of tension stresses in the wear ring. With a straight shoulder as holder surface this minimum sliding movement is likewise possible. In the frictionless state a straight shoulder with an oppositely lying cone surface would also effect a centering of the wear ring with falling temperature. Due to the friction it can therefore be advantageous to additionally attach to the side of the straight shoulder an outer centering shoulder, the shrinking tension of which is still permissible at the lowest arising temperatures. In this way an exact centering at the straight shoulder in each temperature cycle is achieved.
This construction shows advantages in wear rings with a coefficient of thermal expansion xcex1 of less than 10xc3x9710xe2x88x926 degrees C. Wear rings of highly wear-resistant material such as ceramics, for example metal oxides, tungsten carbide or silicon carbide, can be used in this way. Even fluids with larger solid components can be forwarded. In this the diameter of individual solid parts can have the size of the gap width of the sealing gap, since a kind of grinding process for these large parts arises due to the choice of the wear rings. This process has a favorable effect at high operating temperatures above 100xc2x0 C. and/or above 300xc2x0 C., since ceramic materials change their wear properties only at much higher temperatures.